Getting the CPU frequency in C

[Martin Tournoij]

On Mon 10 Sep 2007 00:09, Erik Trulsson wrote:
> On Mon, Sep 10, 2007 at 12:04:45AM +0200, Martin Tournoij wrote:
> > On Sun 09 Sep 2007 22:09, Erik Trulsson wrote:
> > > On Sun, Sep 09, 2007 at 10:50:25PM +0200, Martin Tournoij wrote:
> > > > I'm trying to get the CPU frequency in C:
> > > > 
> > > > #include <unistd.h>
> > > > #include <time.h>
> > > > #include <ctype.h>
> > > > #include <sys/sysctl.h>
> > > > #include <stdio.h>
> > > > #include <sys/time.h>
> > > > 
> > > > int main()
> > > > {
> > > >     int mib[2];
> > > >     size_t size;
> > > >     struct clockinfo clockrate;
> > > > 
> > > >     mib[0] = CTL_KERN;
> > > >     mib[1] = KERN_CLOCKRATE;
> > > >     size = sizeof clockrate;
> > > >     sysctl(mib, 2, &clockrate, &size, NULL, 0);
> > > > 
> > > >     fprintf(stdout, "hz: %i\n", clockrate.hz);
> > > >     fprintf(stdout, "tick: %i\n", clockrate.tick);
> > > >     fprintf(stdout, "spare: %i\n", clockrate.spare);
> > > >     fprintf(stdout, "stathz: %i\n", clockrate.stathz);
> > > >     fprintf(stdout, "profhz: %i\n", clockrate.profhz);
> > > > 
> > > >     return 0;
> > > > }
> > > > 
> > > > I tried to run this on two machines (one machine with hw.clockrate: 1378 and
> > > > the other 797) and it outputs the same on both:
> > > > hz: 1000
> > > > tick: 1000
> > > > spare: 0
> > > > stathz: 133
> > > > profhz: 666
> > > > 
> > > > The profhz value suggest the devil is at work :D although it's probably a some
> > > > stupid mistake on my part :/ Can anyone help?
> > > 
> > > None of the kern.clockrate entries has any particular relationship with the
> > > CPU clock frequency, so it is not unexpected that you would get the same
> > > output from both machines.
> > > 
> > > I think looking at hw.clockrate is the most portable you can get.
> > > If your CPU is using Cool'n'Quiet or the Intel equivalent you can also
> > > look at dev.cpu.N.freq for the current frequency.
> > 
> > I got confused because they both have the same name ... do'h
> > 
> > hw.clockrate doesn't seem to available through C(?),
> 
> Of course it is.  Using sysctlbyname(3) to access it works fine:
> 
> #include <unistd.h>
> #include <sys/sysctl.h>
> #include <stdio.h>
> 
> int main()
> {
>     size_t size;
>     int clockrate;
> 
>     size = sizeof clockrate;
>     sysctlbyname("hw.clockrate", &clockrate, &size, NULL, 0);
> 
>     fprintf(stdout, "hz: %i\n", clockrate);
>     return 0;
> }
> 
> 
> hw.clockrate does however only seem to exist on i386 and amd64 architectures
> so if you are running on something else you will have to find some
> alternative solution. (Parsing the dmesg(8) output?)
> 
> 
> 
> > exec-ing sysctl
> > hw.clockrate would work, but that's not very elegant...
> > 
> > dev.cpu.0.freq doesn't seem to exists on my (Athlon) CPU, it
> > does on my PIII CPU though.
> 
> It partly depends on the CPU, and mostly on the BIOS if the cpufreq(4)
> kernel module will be activated (assuming it has been loaded in the first
> place of course.)

Thought you could only fetch the sysctl values defined in sys/sysctl.h
with sysctl()/sysctlbyname(), apparently not...

Thank you for the help Erik and Mel!